European Patent Application no. EP 1 029 702, discloses a method and arrangement for carrying out a surface treatment using a plasma. The document discloses a variety of embodiments, amongst which are arrangements comprised of a plurality of electrodes forming a discharge space, which electrodes are further arranged for supplying a gaseous substance to the discharge space. The arrangements are further arranged for transporting a film or another medium to be treated through said discharge space. A gas supply provides a gaseous substance to the discharge space in a direction which is substantially perpendicular to the medium to be treated at the location of entrance of the gasstream.
Surface treatment methods and arrangements based on plasma generation are widely used in numerous industries. In photo film industry for instance, similar surface treatment methods are used for preparing thermoplastic polymer films in order to improve the adhesion properties of their surfaces.
A requirement for almost all surface treatment processes is that the surface must be treated by the plasma as homogeneous and uniform as possible. This may be achieved by treating the surface with a stable and homogeneous plasma.
An atmospheric pressure glow plasma is generated by supplying a carrier gas to a discharge space formed between a plurality of electrodes and powering said electrodes using for instance an alternating-current voltage (AC voltage). By transporting a sheet of material to be processed through the discharge space, the plasma can be used for carrying out the surface treatment process. An example of this is the transporting of a polymer film (such as polyethyleneterephtalate (PET), polyethylenenaphtalate (PEN), polytetrafluoroethylene (PTFE), triacetate cellulose (TAC), and the like) over a first electrode through a discharge space formed by said first electrode and one or more second electrodes, whilst supplying a gas to the discharge space and powering the electrodes.
A continuous supply of gas is required in order to maintain the plasma. This may be achieved for instance as described in the above-mentioned document EP 1 029 702, by providing a gasstream through holes or inlets in the second electrodes, such that the gas fills the discharge space adjacent to the material to be treated (present on the first electrode). Although gas is continuously supplied to the discharge space, it has been observed that using a method as described in the above-mentioned document, generating a stable atmospheric pressure glow plasma still provide difficulties.
One of these difficulties, for instance, is that a gas supply as described, which provides a flow of gas which originates from a second electrode and is more or less directed to a first electrode, may give rise to the existence of various flow instabilities, such as vortices, in the discharge space. These instabilities may cause temporal uneven local distributions and density variations of the gas in the discharge space, that may be the cause of instabilities in the generated APG plasma.
Another difficulty, related to the existence of flow instabilities in the discharge space, is the existence of area's in the discharge space that are isolated by the flow (e.g. wakes) due to nearby vortices. In these wakes or area's the supply of fresh gas may be reduced to a minimum, and pollution from numerous sources may build up there. Similar to this is the build-up of pollution in the vortices themselves due to the local pressure minimum and the amount of circulation present in the vortex. Vortices may in fact locally increase the residence time of the flow, increasing the duration over which contaminants may build up in the gas. It may be understood that the build-up of pollution in certain area's of the discharge space may cause the atmospheric pressure glow plasma to be unstable, shortening the uniformity and lifetime thereof and increasing the probability of the occurrence of streamers (filamentary discharges with a short lifetime). This has a negative effect on the surface treatment process.